In recent years, in the area of stereophonics, there is a tendency to express an acoustic field in an up-down direction by adding a speaker on the upper side as well as on a lateral side and a rear side.
On the other hand, not many families install as many speakers as the number of channels in a home theater, and hence a product of virtual surround system (front surround system) that artificially creates a surround acoustic field only with a front speaker is getting mass popularity.
Therefore, it is assumed that few families install a speaker on the upper side as the lateral side and the rear side, and hence a method of artificially creating the speaker on the upper side only with the front speaker is needed in the same manner as the conventional front surround system.
It has been known that peaks and dips appearing on a high frequency side in amplitude-frequency characteristic of a head-related transfer function (HRTF) is a telling clue for a localization of sound of a sound image in the up-down direction and the front-back direction (see, for example, Patent Document 1). It is assumed that these peaks and dips are formed mainly by reflection, diffraction, and resonance due to a shape of an ear.
Further, as shown in FIG. 1, it is indicated that a positive peak P1 appearing near 4 kHz and two notches N1 and N2 appearing first in a frequency band equal to or higher than the frequency band where the peak P1 appears among these peaks and dips have particularly high contribution to the localization of sound in the up-down and front-back directions (see, for example, Non-Patent Document 1).
In this specification, the dip indicates a portion that is recessed in the downward direction compared to a surrounding portion on a waveform diagram such as the amplitude-frequency characteristic of the HRTF. The notch indicates, among the dips, a dip particularly having a narrow width (for example, a bandwidth in the amplitude-frequency characteristic of the HRTF) and a depth equal to or deeper than a predetermined depth, i.e., a sharp negative peak appearing on the waveform diagram.
It is not recognized that the peak P1 depends on a direction of a sound source, and hence the peak P1 appears in the virtually same band regardless of the direction of the sound source. In Non-Patent Document 1, it is considered that the peak P1 is a reference signal used for a human sensory system to search for the notches N1 and N2, and a physical parameter that substantially contributes to the localization of sound in the up-down and front-back directions includes the notches N1 and N2.
Hereinafter, the notches N1 and N2 of the HRTF are referred to as a first notch and a second notch, respectively.